In a wireless communications system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency-Division Multiple Access (FDMA) and Time-Division Multiple Access (TDMA) are the traditional multiple-access schemes for delivering simultaneous services to a certain number of terminals. The basic idea underlying the FDMA and TDMA systems is that of sharing the available resources among several frequencies or several time slots. This is done such that several terminals can function simultaneously without causing interference.
In contrast to those schemes using frequency division or time division, CDMA schemes allow multiple users to share a common frequency and a common time channel by using coded modulation. More precisely, as is well known to the person skilled in the art, a scrambling code is associated with each base station, which makes it possible to distinguish one base station from another. Moreover, an orthogonal code, referred to as the “OVSF code”, is allocated to each remote terminal (such as a cellular mobile telephone, for example). All the OVSF codes are orthogonal to each other, which makes it possible to distinguish one remote terminal from another.
Before sending out a signal on the transmission channel for a remote terminal, the signal has been scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the remote terminal. In CDMA systems, it is further possible to distinguish those which use a separate frequency for transmission and reception (CDMA-FDD system) and those which use a common frequency for transmission and reception, but separate time domains for transmission and reception (CDMA-TDD system).
The invention applies advantageously to the communications systems of the CDMA type, and more particularly to the systems of the CDMA-FDD type. In the remote terminals, such as cellular mobile telephones, a single power amplifier is typically used for transmission. This power amplifier generally has a wide range of radio frequency power operation. Further, the power amplifier may continually be in operation during the communications, which is particularly true in CDMA-FDD systems.
Moreover, the transmission power delivered by the power amplifier can vary within a predetermined range of powers, typically from −50 dBm to 24 dBm in the case of third-generation mobile telephones. Within this power range, the transmission power is adjusted on the basis of power information received regularly by the telephone and originating from the base station. The power amplifier is currently designed to exhibit the greatest efficiency for the maximum transmission power.
In contrast, for intermediate or low powers, a significant deterioration in the efficiency occurs. This is because the quiescent current of the power amplifier does not change while the transmitted power decreases. Thus, in such operating modes at low or intermediate power, the efficiency (i.e., yield) decreases drastically to a value on the order of a few percent (e.g., 1 to 5%). Thus, a loss of energy results within the battery, which reduces its life span.